Experiment-based studies on the effects of hydrogen on strength of materials are being carried out in the Hydrogen Materials Compatibility Research Division at the Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University. Particularly for fatigue e limit in high-pressure hydrogen is studying at the Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS). The studies are doing with the students from the Department of Mechanical Engineering by the help of the Department.

Education: I am in charge of Academic frontier for all undergraduate students, Advanced engineering for international course students, mechanical engineering experiment for 3rd grade student in mechanical engineering course, and hydrogen structural material for master course students in mechanic al engineering. For graduation thesis, I am aiming at not only cutting-edge research but also that students will be able to manage independently. There the students will learn knowledges about strength of material as well as programing, electric circuit, machining, design, drawing, presentation and so on. For the improvement of my teaching skills, I attend to faculty development courses and join to Japanese Society for Engineering Education.

Research: Metal fatigue is the major subject of my researches. Particularly, fretting fatigue is my lifetime work. At present, I put the most effort into the research on the effects of hydrogen on material strength. (1) Effect of hydrogen on fretting fatigue, fatigue and fracture toughness: To achieve optimization of cost and performance of high-pressure hydrogen containment systems without compromising safety, these studies are being carried out.

(2) Fatigue limit in high-pressure hydrogen gas: We developed a special fatigue testing machine to obtain fatigue limit in high-pressure hydrogen. We have many strong request for such data from industry and the government.

(3) Material issues in next-generation hydrogen utilization systems: It can be expected that SOFC and SOEC will be extensively used in the near future. Therefore, material strength in hydrogen at elevated temperature must be investigated.

(4) Crack initiation in fretting fatigue: Fretting fatigue is one of the major factors in the design of machines and structure to prevent catastrophic failure accidents. Mechanism and the method how to evaluate quantitatively are studying with a company.

Social contribution: Steering committee of Kyushu branch of the Japan Society of Materials Science (2008-2015), Editorial committee of journal of Japan Society for Mechanical Engineers (2009-2010), Referee of contributed paper to JSME journal (domestic and international), International symposium (MMYoung and ISFF), Workshops, Board member of the Japan Society of Materials Science (2016-)

International cooperation: Local committee (Asia region) of International symposium on fretting fatigue (ISFF 6- 8), Planning committee of international symposium for young researchers (M&M div, JSME)

Effect of hydrogen gas and absorbed hydrogen on fretting fatigue strengthkeyword : Hydrogen gas, Hydrogen concentration, Fretting fatigue strength2003.04The objective of this study is to clarify the effect of hydrogen gas on fretting fatigue strength of the materials used for hydrogen utilization machines and structures. In hydrogen gas environment, fretting fatigue strengths of A286, SUS304 and SUS316L decrease in the long-life region..

Selection of shape and size of stress-relief groove for improvement of fretting fatigue strenthkeyword : Fretting fatigue, Stress-relief groove, Improvement of fretting fatigue strength2005.04Stress relief groove has been used to improve the fretting fatigue strength of fitted part between mechanical components. However, the applicability of groove has not been fully evaluated and there are insufficient investigations to determine the optimal groove shape. In this study, the evaluation of fretting fatigue strength of specimens which have various shapes of stress relief groove was conducted by fretting fatigue tests and FEM analyses in order to develop an index for the selection of groove shape..

Current and Past Project

Introduction of nitrogen stainless steel and its weld joint is required for high-pressure hydrogen piping. Nitrogen is important alloying element to improve strength and hydrogen compatibility, however, the state of existence may be changed during welding process. This study is aiming at development of weld joint of nitrogen stainless steel which is applicable for high-pressure hydrogen piping. ①Development of welding technique, ②Characterization of microstructure, ③Study on hydrogen compatibility of developed weld joint.

久保田 祐信, 片岡 俊介, 髙﨑 大裕, 近藤 良之, A Quantitative Approach to Evaluate Fretting Fatigue Limit Using a Pre-Cracked Specimen, Tribology International, http://dx.doi.org/10.1016/j.triboint.2016.10.017, 108, 48-56, 2017.04, A pre-cracked specimen, which has a 70-μm-deep crack, was used for the fretting fatigue test to understand the reasons for the change in the fretting fatigue limit due to changes in the contact pressure, position of the precrack, and foot length of the contact pad. The threshold stress intensity factor range to crack propagation of the pre-crack, ΔKth, was obtained by the crack growth test. The stress intensity factor range of the pre-crack under fretting conditions was then evaluated by a finite element analysis to estimate the fretting fatigue limit of the pre-crack specimen. The effects of these variables on the fretting fatigue limit were quantitatively explained by the results of the FEM and ΔKth of the short crack..

3.

久保田 祐信, 薦田 亮介, Jader Furtado, Fretting fatigue in hydrogen and the effect of oxygen impurity, Proc. the Asian Conference on Experimental Mechanics 2016 (ACEM 2016), 2016.11, Fretting is a coupled problem of fatigue and frictional contact. It brings unique phenomena that enhance the hydrogen-induced degradation of fatigue strength. Therefore, the role of hydrogen in the fretting fatigue is seriously considered by both manufacturers and users of hydrogen equipment. The fretting fatigue limit in hydrogen was significantly lower than that in air, whereas the fatigue limit of the conventional fatigue test was the same for the both tests. The results clearly demonstrate that the fretting had some specific effects that enhance hydrogen-induced degradation of fatigue strength..

Fundamental Mechanisms Causing Reduction in Fretting Fatigue Strength by Hydrogen(Effect of Hydrogen on Small Crack Initiation at the Adhered Spot).

6.

Fundamental Mechanisms Causing Reduction in Fretting Fatigue Strength by Hydrogen(Effect of Hydrogen on Small Crack Initiation at the Adhered Spot).

7.

"Effects of Multiple Overloads and Hydrogen on High-Cycle Fatigue Strength of Notched Specimen of Austenitic Stainless Steels" in the transactions of the Japan Society for Mechanical Engineers, Ser. A.

8.

Kanetaka MIYAZAWA, Masato MIWA, Akihiro TASHIRO, Tatsuro AOKI Masanobu KUBOTA and Yoshiyuki KONDO, Improvement of Torsional Fretting Fatigue Strength of Splined Shaft Used for Car Air Conditioning Compressors by Hybrid Joint, Journal of Solid Mechanics and Materials Engineering, 10.1299/jmmp.5.753, 5, 12, 753-764, 2011.12, To improve the fatigue strength of the splined shaft used for a car’s air conditioning compressor, press fit was added to the innermost part of the spline. This shaft connection consisting of a spline and press fit is called a "hybrid joint" in this study. A torsional fretting fatigue test was performed focusing on the effect of the amount of interference on the fatigue strength. The fatigue strength of the splined shaft was drastically increased by the hybrid joint. The fatigue strength of the hybrid joint was at most 8 times higher than that of the conventional spline-joint shaft. The fatigue strength as well as the failure mode of the hybrid-jointed specimens were changed depending on the amount of interference. The reason was that the relative slip was significantly reduced with an increase in the amount of interference. The specimen consisted of a shaft, a boss and a bolt. The hybrid joint prevented loosening of the bolt, while loosening of the bolt was found to occur in the conventional spline-joint shaft..

9.

Mechanism of reductin of fretting fatigue limit in hydrogen gas in SUS304.

Masanobu Kubota, Ryosuke Komoda, Jader Furtado, The effect of hydrogen on fretting fatigue strength of austenitic stainless steels, The 9th National Conference on Pressure Vessel Technology (NCPVT), 2017.11, Fretting fatigue is a form of contact fatigue, which frequently occurs on the mating surfaces of joined structures subjected to a fatigue load, which could be one of the major concerns in the design of machines and structures due to the significant reduction in the fatigue strength. Fretting fatigue strength of austenitic stainless steels was significantly reduced in hydrogen gas. Since hydrogen could influence both the fatigue and the phenomena occurring at the contacting surfaces such as friction, fretting wear, oxidation, etc., the mechanisms that causes the reduction in the fretting fatigue properties are very complicated. When considering the service conditions of hydrogen-containment systems, some amount of impurities in the hydrogen should be accepted. The objective of this study is to clarify the effects of hydrogen and oxygen addition to hydrogen on the fretting fatigue strength. For the fretting fatigue test, a controlled method to add ppm-level oxygen to a hydrogen environment was firstly established. The fretting fatigue tests were carried out in high-purity hydrogen (0.088 vol. ppm O2) and in oxygen/hydrogen mixtures with 5, 35 and 100 vol. ppm O2 concentrations. The material was JIS SUS304 austenitic stainless steel. The fretting fatigue strength was significantly lower in hydrogen gas than in air. Fretting removed surface oxide layer. It caused two primary consequences, enhancement of hydrogen entry and local adhesion between contacting surfaces. The adhesion resulted severe cyclic plastic strain at the adhered spot. It facilitated formation of small cracks. In addition, microstructural change from austenite to martensite was also induced by the cyclic plastic strain. The reduced fretting fatigue strength in hydrogen was synergistically caused by chemical, physical and mechanical processes. The fretting fatigue strength in the oxygen/hydrogen mixtures was significantly lower than that in the high-purity hydrogen. Since addition of oxygen to hydrogen environment usually mitigate the effect of hydrogen, the results obtained were opposite. The fretting fatigue strength in the oxygen/hydrogen mixtures slightly changed depending on the oxygen level. Based on the X-ray photoelectron spectroscopy analysis of the fretted surface, it was found that an oxide layer was produced on the fretted surface in the oxygen/hydrogen mixture by overcoming the removal action of the fretting. It resulted weakening of the adhesion between contacting surfaces, and larger slip between contacting surfaces was produced. As the result, stress conditions at the contact part were changed. As the result that the oxidation behavior was changed, the fretting fatigue strength was reduced by the addition of oxygen..

2.

久保田 祐信, MACADRE ARNAUD PAUL ALAIN, Hydrogen Compatibility of Ultra-Fine Grain Austenitic Stainless Steel, International Colloquium on Environmentally Preferred Advanced Generation Grid evolution global summit "HYDROGEN" (ICEPAG2017), 2017.03, A certain kind of austenitic stainless steel shows a good resistance against hydrogen-induced degradation of material strength. However, basically the yield strength of single austenitic phase materials is not so high. For hydrogen equipment, high strength is very beneficial in terms of cost of the material and performance of the equipment. Therefore, the development of high-strength austenitic stainless steels is strongly desired. Alloying and following precipitation hardening are useful to obtain high strength, but these methods are not always good in terms of material cost. On the other hand, grain refinement is a promising method to improve the mechanical strength of metals. The idea of this study is the combination of austenitic stainless steel and grain refinement in order to obtain both high strength and hydrogen compatibility.The material was synthesized by a thermo-mechanical treatment invented by Takaki et al [Takaki, 1988]. The grain size was adjusted during reaustenization of the microstructure so that its average size was 1 m. The yield strength was 596 MPa. It was almost twice higher than that of commercial austenitic stainless steels. The improvement of the yield strength by the grain refinement was significant.Fatigue tests were carried out to obtain the fatigue strength and identify the crack origin. In addition, fatigue crack growth tests were also carried out to obtain the crack growth behavior. The effect of hydrogen on the fatigue properties was evaluated with hydrogen charged material. The reference materials were JIS SUS316 and JIS SUH660. The fatigue limit of the 1 m grain size material was approximately 2.8 times higher than that of the SUS316. The hydrogen charge didn’t affect the fatigue limit, but reduced the fatigue life in the short life region. The crack origin was a non-metallic inclusion. It was surprising that the size of the inclusion was 2 m. Usually an inclusion acts as the crack origin when the material hardness is higher than HV400 and its size is several tens of m. The hardness of the 1 m grain size material was less than HV300. Therefore, the experimental fact that the crack initiated from a 2 m inclusion was beyond the knowledge about engineering metals. Generally, the crack growth resistance is deteriorated by grain refinement because the crack path becomes smooth. However, this material showed a good crack growth resistance which was equivalent to the SUH660. It was found that the crack tip opening displacement (CTOD) during crack growth was distinctively smaller compared to those of the reference materials. Furthermore, the plastic zone size at the crack tip was also considerably smaller than those of the reference materials. According to classic crack growth mechanisms, the CTOD is proportional to the plastic zone size and the CTOD governs the crack growth rate. Therefore, a possible mechanism for the good crack growth resistance is the barrier of the grain boundary of the small grains against the dislocation motion and the following suppression of plastic deformation at the crack tip. The hydrogen charge caused an acceleration of the crack growth, and this result is consistent with the reduced fatigue life of the hydrogen charged material. The reason for the acceleration of the crack growth was the transformation of microstructure from austenite to strain-induced martensite during crack growth. The grain refinement significantly improved the both static and fatigue strength. However, there is room for improvement in hydrogen compatibility..

3.

久保田 祐信, 薦田 亮介, Jader Furtado, Fretting fatigue in hydrogen and the effect of oxygen impurity, The 15th Asian Coference on Experimental Mechanics (ACEM2016), 2016.11, Fretting is a coupled problem of fatigue and frictional contact. It brings unique phenomena that enhance the hydrogen-induced degradation of fatigue strength. Therefore, the role of hydrogen in the fretting fatigue is seriously considered by both manufacturers and users of hydrogen equipment. Figure 1 shows the result of the conventional fatigue test and the fretting fatigue test of SUS304 austenitic stainless steel in 0.1 MPa hydrogen gas and air. The fretting fatigue limit in hydrogen was significantly lower than that in air, whereas the fatigue limit of the conventional fatigue test was the same for the both tests. The esults clearly demonstrate that the fretting had some specific effects that enhance hydrogen-induced degradation of fatigue strength..

4.

髙﨑 大裕, 久保田 祐信, 薦田 亮介, 吉田 修一, 奥 洋介, 牧野 泰三, 杉野 正明, Effect of Contact Pressure on Fretting Fatigue Failure of Oil-Well Pipe Material, The 15th Asian Coference on Experimental Mechanics (ACEM2016), 2016.11, Fretting fatigue is a combination of fatigue and a kind of wear. Since the fretting fatigue strength is significantly lower than plain fatigue strength, fretting fatigue is one of the most important factors in the design of components. In resent oil-well development, drilling casing technology becomes popular. As shown in Fig. 1, the thread joint between pipes might suffer from fretting fatigue. In the results of full-scale fatigue test of thread joint, there were two failure modes [1]. The first one was the fatigue failure at the thread corner radius, which has been already quantitatively evaluated [2]. The second one is the fretting fatigue failure at the inner contact surface, which has not been studied yet. The objective of this study is to clarify the mechanism of the fretting fatigue failure at the inner contact surface. For this purpose, a fretting fatigue test with different contact pressure was carried out..

5.

久保田 祐信, Akihide Nagao, University of Illinois at Urbana Champaign, Harvard University, University of Illinois at Urbana Champaign, University of Thessaly, Sandia National Laboratories, Livermore, SOFRONIS PETROS, Constitutive equations of hydrogen-enhanced plasticity for quantitative understanding of the mechanisms of hydrogen-assisted fracture, 2016 International Hydrogen Conference, 2016.09, Although the phenomenon of hydrogen-induced degradation of metals and alloys is well documented, there remains a paucity of information with regard to 3-D constitutive material model in the presence of hydrogen. Such constitutive material models that account for the hydrogen effects on the microplasticity of crystals are central to the challenges for material performance prognosis under in-service conditions. Once such a constitutive model is developed and tested under monotonic and cyclic loading conditions, it can be used in a general purpose finite element code to investigate such issues as the role of maximum stress in promoting accelerated fatigue crack growth in the presence of hydrogen.The objective of this work is the development of a single crystal plasticity model that accounts for the hydrogen effect on dislocation activity along individual slip systems and the interactions between slip systems under varied crystal orientations and loading triaxialities. Toward this objective, a concerted experimental and simulation effort has been mounted by an international team of researchers from the academia, industry, and national laboratories.In the experimental part of this study, a special tensile test fixture has been designed to carry out uniaxial tension, which can accommodate rotation of the tensile axis and allow for tension free from bending moments. The shape of the specimen has been chosen using computational plasticity so as to minimize stress concentration effects close to the specimen grips. To monitor the response of specific crystal orientations the specimen is mounted and stressed in a way that slip proceeds sequentially from a given single system to multiple slip systems as the load increases. A non-contact strain measurement system is employed in order to avoid imposing any deformation constraints on the specimen. The material used for experiments in this study is single crystal nickel, which has a FCC structure. The computational part of the study involves the development of a single crystal plasticity model accounting for the hydrogen effects on slip along individual systems and their interactions. At the preliminary stages of this investigation, the model of Bassani and Wu has been adopted and modified to account for the hydrogen effects. .

6.

久保田 祐信, 薦田 亮介, Jader Furtado, The effect of oxygen impurities on fretting fatigue of austenitic stainless steel in hydrogen gas, 2016 International Hydrogen Conference, 2016.09, Fretting fatigue is a form of contact fatigue, which frequently occurs on the mating surfaces of joined structures subjected to a fatigue load, which could be one of the major concerns in the design of machines and structures due to the significant reduction in the fatigue strength. For the fretting fatigue in hydrogen, a significant reduction in the fretting fatigue strength has been reported in austenitic stainless steels. Since hydrogen could influence both the fatigue and the phenomena occurring at the contacting surfaces such as friction, fretting wear, oxidation, etc., the mechanisms that cause the reduction in the fretting fatigue properties are very complicated. When considering the service conditions of hydrogen-containment systems, some amount of impurities in the hydrogen should be accepted. The objective of this study is to clarify the effect of oxygen added to hydrogen on the fretting fatigue strength of an austenitic stainless steel. For the fretting fatigue test, a controlled method to add ppm-level oxygen to a hydrogen environment was established. The fretting fatigue tests were carried out in high-purity hydrogen (0.088 vol. ppm O2) and in oxygen/hydrogen mixtures with 5, 35 and 100 vol. ppm O2 concentrations. The material was JIS SUS304 austenitic stainless steel. The fretting fatigue strength in the oxygen/hydrogen mixtures was significantly lower than that in the high-purity hydrogen. The fretting fatigue strength in the oxygen/hydrogen mixtures slightly changed depending on the oxygen level. Based on the X-ray photoelectron spectroscopy analysis of the fretted surface, it was found that the oxide layer of the stainless steel was removed by the fretting in the high-purity hydrogen. The removal of the oxide layer could contribute strong adhesion between contacting surfaces. On the other hand, an oxide layer was produced on the fretted surface in the oxygen/hydrogen mixture by overcoming the removal action of the fretting. It resulted weakening of the adhesion between contacting surfaces, and larger slip between contacting surfaces was produced. As the result, stress conditions at the contact part were changed. Therefore, the change in the oxidation behavior is closely related to the reduction of the fretting fatigue strength by the addition of oxygen..

7.

久保田 祐信, 片岡 俊介, 近藤 良之, A quantitative approach to evaluate fretting fatigue limit using a pre-cracked specimen, 8th International Symposium on Fretting Fatigue (ISFF8), 2016.04, As the general feature of fretting fatigue, non-propagating cracks are frequently found in the unbroken specimens or structural members of machines. Based on this fact, the fretting fatigue limit can be evaluated by the critical stress to crack propagation of a small crack under fretting conditions. In this context, a pre-cracked specimen was prepared for the fretting fatigue test. In the fretting fatigue test using a pre-cracked specimen, the fretting fatigue limit decreased, then increased with an increase in the contact pressure. The threshold stress intensity factor to crack propagation of the small crack, ΔKth, was obtained by the crack growth test. The stress intensity factor range of the pre-crack under fretting conditions, ΔK, was evaluated by a finite element analysis. It was clarified that the change in the fretting fatigue limit with a change in the contact pressure is quantitatively explained by comparison of ΔK to ΔKth. Based on this result, the effect of the position of the pre-crack on the fretting fatigue limit was also both experimentally and analytically investigated..

8.

久保田 祐信, 森 功一, MACADRE ARNAUD PAUL ALAIN, Study on Hydrogen Compatibility in Fatigue of Ultra-Fine Grain Austenitic Stainless Steel, European Congress and Exhibition on Advanced Mterials and Processes 20415 (EUROMAT 2015), 2015.09, This study is aiming to develop higher yield strength austenitic steels without compromising performance in hydrogen by a method of grain refinement. Austenitic stainless steels are superior in hydrogen compatibility than ferritic and martensitic steels. However, austenitic steels have the drawback of relatively low proof strength. It causes increase of thickness of tubes, and it results increase cost and decrease of flow rate. The material was prepared by thermo-mechanical treatment using reversion from strain-induced martensite to austenite, which has been developed by Takaki et al [1]. Two materials, which have 1m and 21m of average grain size, were prepared. The microstructure is shown in Fig.1. The yield strength was 596MPa for the material with 1m grain size and the proof strength was 156MPa for the material with 21m grain size.Fatigue test of hydrogen charged and uncharged materials was done with a stress ratio of -1 at a loading frequency of 15Hz in air. Crack growth tests were also performed. The fatigue limit of the 1m grain size material was significantly higher than that of the 21m grain size material. But the fatigue life in the short life region was significantly reduced by hydrogen charge. It seems that there is no precedent, the crack growth resistance of the ultra-fine grain material was good compared to that of commercial austenitic stainless steels. However, the hydrogen charge accelerated crack growth. This was consistent with the reduction of fatigue life. Based on the observation of morphology of crack, slip bands and crack opening displacement, a possible toughening mechanism, which is inherent in ultra-fine grain austenitic stainless steel was considered..

9.

久保田 祐信, 薦田 亮介, Jader Furtado, Basic Study on The Effect of Hydrogen on Fretting Fatigue, Society of Tribologists and Lubrication Engineers (STLE), 2014 Annual Meeting at Disney Contemporary Resort, 2014.05, The effect of hydrogen on the fretting fatigue strength of SUS304 austenitic stainless steels and the mechanisms were studied. The fretting fatigue strength is significantly lower in hydrogen gas than in air. The fretting fatigue strength is decreased by hydrogen charge. Addition of oxygen partially increased the fretting fatigue strength in gaseous hydrogen. Adhesion between the contacting surfaces is one of the causes of the reduced fretting fatigue strength in hydrogen. The adhesion causes increase in stress, severe plastic deformation and microstructure change.Initiation limit of fretting fatigue cracks was reduced by hydrogen. This is also one of the causes of the reduced fretting fatigue strength in hydrogen..

10.

Factors affect fretting fatigue propertoes in hydrogn.

11.

久保田 祐信, Jader Furtado, 薦田 亮介, 吉開 巨都, Fretting fatigue properties under the effect of hydrogen and the mechanisms that cause the reduction in fretting fatigue strength, Joint HYDROGENIUS & I²CNER International Workshop on Hydrogen-Materials Interactions, 2014.01, The effect of hydrogen on the fretting fatigue strength of austenitic stainless steels and the mechanisms were studied. The fretting fatigue strength is significantly lower in hydrogen gas than in air. The fretting fatigue strength is decreased by hydrogen charge.Adhesion between the contacting surfaces is one of the causes of the reduced fretting fatigue strength in hydrogen in terms of stress concentration, severe plastic deformation and microstructure change..

12.

久保田 祐信, High-cycle fatigue properties of work-hardened copper in 10MPa hydrogen gas, Joint HYDROGENIUS & I²CNER International Workshop on Hydrogen-Materials Interactions, 2014.01, High-cycle fatigue properties of work-hardened oxygen-free copper in 10MPa hydrogen gas were studied. (1) The fatigue limit in hydrogen increased compared to that in air.(2) The increased fatigue strength was caused by the delay of the crack growth.(3) Hydrogen participated in the slip deformation. Hydrogen activates dislocations which are immobile in air.(4) Crack initiation and development of slip bands occurred at lower stress amplitude in hydrogen than in air. The possible mechanism is HELP..

Jader Furtado, 薦田亮介, 久保田 祐信, Fretting fatigue properties under the effect of hydrogen and the mechanisms that cause the reduction in fretting fatigue strength, 13th International Conference on Fracture (ICF-13), 2013.06, Fretting fatigue, which is a composite phenomenon of metal fatigue and friction, is one of the major factors in the design of mechanical components as it significantly reduces fatigue strength. Since hydrogen can influence both fatigue and friction, fretting fatigue is one of the important concerns in designing hydrogen equipment. The authors carried out the fretting fatigue tests on austenitic stainless steels in order to characterize the effect of hydrogen and to explain the mechanism responsible for hydrogen embrittlement. In this study, the significant reduction in fretting fatigue strength due to hydrogen is shown including other factors influencing the fretting fatigue strength such as surface roughness, hydrogen content and the addition of oxygen. The cause of the reduction in the fretting fatigue strength in hydrogen is local adhesion between the contacting surfaces and subsequent formation of many small cracks. Furthermore, hydrogen enhances crack initiation under fretting fatigue conditions. Transformation of the microstructure from austenite to martensite is another possible reason. A hydrogen charge also reduces the fretting fatigue strength. The cause is the reduction in the crack growth threshold, Kth, due to hydrogen..

15.

宮澤金敬, 三輪昌人, 近藤 良之, 久保田 祐信, DEVELOPMENT OF THE HYBRID JOINT AND TORSIONAL FRETTING FATIGUE STRENGTH IMPROVEMENT IN THE POWER TRANSMISSION SHAFT, Seventh International Symposium on Fretting Fatigue, 2013.04, Downsizing and reduction of the weight of components is a critical issue for automotive parts manufacturers in the face of strong demand for a reduction of the environmental burden caused by automobiles. Improving the efficiency of the car air conditioning compressor significantly contributes to improved fuel efficiency, but requires downsizing of the compressor’s power transmission shaft. At the same time, the fatigue strength of shaft has to be improved in order to transmit a given level of power through a smaller component. Currently, the spline used for the compressor transmission shaft is the part most prone to fretting fatigue, so an investigation was carried out revealing that the fretting fatigue strength of the downsized spline was insufficient to meet design requirements. Therefore, a new type of joint, a hybrid joint, was developed that combined the press-fit and spline. The structure of the hybrid joint is shown in Fig. 1. The hybrid joint prevented fretting fatigue failure of the downsized spline, resulting in a significant improvement in fatigue strength. In order to use the hybrid joint in the actual product, factors with a bearing on its fatigue strength, particularly the contact pressure and the contact length in the press-fit part, were studied. Contact pressure is one of the major factors influencing fretting fatigue strength of the press-fit part. Since the diameter of the shaft used in this study is relatively small, the contact pressure significantly changes by the degree of interference, which is the difference between shaft diameter and inner diameter of boss..

16.

久保田 祐信, 薦田亮介, 足立裕太郎, 近藤 良之, Jader Furtado, EFFECT OF HYDROGEN AND IMPURITIES ON FRETTING FATIGUE PROPERTIES, Seventh International Symposium on Fretting Fatigue, 2013.04, The authors have reported that the reduction in fatigue strength due to fretting can be significantly enhanced in hydrogen. In this study, the mechanisms enhancing hydrogen embrittlement by fretting were investigated. Impurities in hydrogen can be considered as an influencing factor on the fretting fatigue strength in hydrogen. The effect of oxygen addition was also studied..

17.

薦田亮介, 久保田 祐信, 近藤 良之, Jader Furtado, THE MECHANISM CAUSING REDUCTION IN FRETTING FATIGUE STRENGTH DUE TO HYDROGEN, Seventh International Symposium on Fretting Fatigue, 2013.04, Hydrogen is the most promising candidate as a new energy carrier in the very near future. Fretting fatigue in hydrogen containment equipment should be considered in order to ensure safety. The authors have reported a significant reduction in the fretting fatigue strength due to hydrogen [1]. One of the causes of the reduced fretting fatigue strength in hydrogen is small cracks that emanate from a locally adhered spot between contacting surfaces. The objective of this study is to establish a quantitative understanding of the effect of hydrogen on the initiation of the small cracks under fretting fatigue conditions..

18.

A study on the damage caused by repetitive open-close movement in a high-pressure hydrogen gas valve.

19.

Effects of Multiple Overloads and Hydrogen on High-Cycle Fatigue Strength of Notched Specimen of Austenitic Stainless Steel and Prediction of Reduction of Fatigue Limit.

20.

Effects of Notch Root Radius and Stress Ratio on the Behavior of Short Crack at Notch Root.

Development of environmental-friendly and energy-saving car air-conditioning compressor using downsized power transmission shaft by hybrid joint structure

Mechanism of Reduction of Fretting Fatigue Limit in Hydrogen Gas in SUS304

Award for the Best Paper 2007, Japan Society of Materials ScienceEffect of stress relief groove shape on fretting fatigue strength and index for the selection of groove shape

Studies on effect of influence factors on fretting fatigue and quantitative evaluation of fretting fatigue strength

Educational

Educational Activities

I make research plans for students by fully utilizing international and interdisciplinary research environment of I2CNER. I provide research guidance for doctor, master and undergraduate students. One of the important points in my educational concept is that students will develop ability to perform their studies with self-management. To do this, I make a great effort to back up the students in various aspects.

A lecture of hydrogen energy structural materials is opened for master course's students. I provide basic knowledge and information about advanced studies related to the effect of hydrogen on strength of metal materials to the students who want to work in hydrogen society. I have an experiment for 3rd grade students. The subject is torsion test of mild steel and cast iron. This experiment is important to enhance understanding of classroom lectures such as strength of materials, elastic mechanics and engineering materials. For new comer students, Advanced Engineering (English) and Academic Frontier (Japanese) are opened. Importance of use of hydrogen energy and materials issues are introduced.

Math and linear algebra were held in Kyushu Politechnique College until 2006. A lot of examples related with engineering problems are included in order to build up interest.

As an extensive education activities, I have been served several workshops and seminars concerning fretting fatigue, fatigue and hydrogen-materials interactions. Most of the participants are from manufacturing companies. Based on these activities, I have a lot of collaboration with industry.

Social

Professional and Outreach Activities

Many collaborations with industry in terms of analysis and prevention of failure caused by fatigue, fretting fatigue and hydrogen embrittlement were done. Usually failure accident is hidden by the sense of company secret, but in two cases, publications in scientific journal were made. The results of my research were taken into account in some lawsuits as an expert opinion.

For the contribution to regional industry, some collaborative researches were done with small companies in Kyushu area.

I believe that development of real products is the highest goal of our study (Technology transfer). It is very important in terms of relevance of studies.

I served as a board member, journal editors committee, reviewer, managing committee in academic societies. I try to do my best to further development of academic societies.

I held many seminars for citizens, high-school students and engineers in order to educate importance of fatigue, fretting fatigue and hydrogen embrittlement..